1. Field of the Invention
The invention relates to clutches and, more particularly, to hydraulically operated clutches taking measures to reduce the rate of clutch engagement, thereby reducing noise and shift shock.
2. Discussion of the Related Art
Hydraulically operated friction clutches are well known and are used in a variety of applications. For example, they are often used in the transmissions of four wheel drive vehicles to selectively couple a drive spider to one or both of the drive axles. Such clutches selectively couple a rotatable drive element (typically a shaft coupled directly to the drive spider) to a rotatable driven element (typically a pinion coupled to an output shaft either directly or through other clutches). The typical clutch includes a clutch plate stack, a stationary backplate, and a movable piston. The clutch plate stack is formed from a plurality of interleaved clutch plates extending between the drive element and the driven element. The piston is hydraulically actuatable to slide axially upon demand to compress the clutch plate stack and engage the clutch.
In the simplest hydraulically operated clutch, the entire axial surface of the piston facing away from the clutch plate stack is subjected to a uniform system pressure throughout clutch engagement. Clutches of this type are relatively inexpensive to manufacture because they do not require stepped pistons, multiple bores of various sizes supplying fluid to the pistons, or more than two seals. However, clutches of this type exhibit the disadvantage of locking up or becoming fully clamped or engaged very rapidly. Rapid engagement applies substantial shock to the driven components of the clutch and downstream devices (commonly referred to as "shift shock"), causing substantial noise or chatter and contributing to relatively rapid wear of clutch components.
Proposals have been made to reduce shift shock by reducing the rate of pressure increase in the clutch after initial engagement, thereby reducing the rate at which the percentage of torque transferred from the drive element to the driven element increases from zero to 100%. The simplest solution to the shift shock problem is to provide a restriction reducing the rate of fluid flow into the clutch actuating chamber. However, restricting the flow of fluid into the clutch chamber is only a partial solution to the problem because restricting fluid flow sufficiently to appreciably reduce shift shock also necessarily delays initiation of clutch engagement. Such delays are undesirable because most operators demand an immediate response to a clutch engagement command.
Another solution to the problem of shift shock is disclosed in U.S. Pat. No. 3,470,988 to Sieverkropp. The clutch disclosed by the Sieverkropp Patent employs a stepped actuating piston which presents a dual actuating chamber. A relatively small or inner portion of the piston freely communicates with a source of actuating fluid and thus is subjected to system pressure almost instantaneously upon generation of a clutch engagement command. A relatively large or upper surface of the piston 1) communicates with the source of fluid pressure only through a restricted feed passage and 2) has an axial passage formed therethrough. According to the Sieverkropp specification, upon initial movement of the actuating piston, air rushes through the axial passage from the chamber containing the clutch plate stack and into the actuating chamber where it is trapped. The trapped air within the actuating chamber is said to provide a cushioning effect due to the fact that the air is somewhat compressible as compared to the oil.
The Sieverkropp clutch exhibits distinct drawbacks and disadvantages. Most notably, it is relatively expensive to manufacture for at least three reasons. First, the piston and the piston carrier must be formed with steps in them. Second, at least three sliding seals must be provided--a first to seal the inner radial end of the stepped piston with respect to the drive shaft, a second to seal the outer radial end of the stepped piston with respect to the piston carrier, and a third to seal the lower portion of the stepped piston from the upper portion, thereby providing the distinct chambers required for acceptable operation. Third, two distinct flow passages must be machined, one of which is substantially restricted, to permit the required separate supply of fluid to the first and second surfaces of the piston.